1. Field of the Invention
This invention relates to an image recording apparatus which is able to record images on both sides of a recording sheet, and to a sheet conveying method thereof.
2. Description of the Related Art
A conventional image recording apparatus which is able to record images on both sides of a recording sheet a(hereinafter referred to as "a both-side recording apparatus", when applicable) will be described with reference to the accompanying drawings.
In FIG. 4, reference characters 1a, 1b and 1c designate sheet supply sections which accommodates recording sheets; and 2, a photo-sensitive drum which starts rotating in response to a signal from a controller (not shown). As the photo-sensitive drum 2 starts rotating, the surface of the photo-sensitive drum 2 is uniformly charged by a corona charger (not shown). Further, an electro-static latent image is formed on the charged photo-sensitive drum 2 by an optical system (not shown). When reaching a developing unit, the latent image thus formed is developed as a toner image (visible image) on the photo-sensitive drum 2. The toner image thus formed by the conventional electro-photo process, is transferred onto a recording sheet by a transferring unit 3 which is delivered from the sheet supply section 1a, 1b or 1c. Further in FIG. 4, reference numeral 4 denotes a fixing unit which is adapted to fix the toner image which has been transferred onto the recording sheet; 10, a flap which is to control the direction of conveyance of a recording sheet; and 9, a sheet discharge section, namely, a sheet discharge tray on which a recording sheet on which an image has been formed and fixed is placed.
In the following description, for simplification in description, a sheet conveying path 12 connecting the sheet supply sections 1a, 1b and 1c, the image forming means 2, 3 and 4, and the discharge sheet tray 9 will be referred to as "a sheet sending path", when applicable. Reference numeral 11 designates a pull-in path, branching from the sheet sending path 12 downstream of the image forming means 2, 3 and 4 with respect to the direction of conveyance of a sheet. The pull-in path selectively pulls in a recording sheet which is delivered from the fixing unit 4 by switching the flap 10. Reference numeral 14 designates a return path which branches from the middle of the pull-in path 11, and its end meets the sheet sending path 12 upstream of the image forming means 2, 3 and 4 with respect to the direction of conveyance of a sheet. In the case where it is required to record images on both sides of a recording sheet, the sheet delivered from the fixing unit 4 is pulled in the pull-in path 11. The sheet thus pulled in is sent to the return path 14, so that the sheet on one side of which the image has been recorded is sent to the image forming means again, whereby the aimed both-side image recording operation is carried out. Furthermore, in FIG. 4, reference numerals 5 and 6 designate conveying rollers provided before and after the flap 10; 7, conveying rollers which is provided on the pull-in path 11 in such a manner that they can be turned in the forward -direction and in the reverse direction in a switching mode; and 13, a flap to send the sheet pulled in the pull-in path 11 to the return path 14 or to return it to the sheet sending path 12 again so as to be discharged into the discharge sheet tray 9.
In the case where, in the both-side recording apparatus shown in FIG. 4, the recording sheet is guided to the pull-in path 11 and turned over in the one-side print mode; in the case of a so called "one-side print, face-down sheet discharge mode", as shown in FIGS. 6A to 6D, the sheet P1 is pulled in with the conveying rollers 7 as shown in FIG. 6A, immediately after the rear end of the sheet P1 reaches the position A as shown in FIG. 6B, the direction of rotation of the conveying rollers 7 is switched, and as shown in FIG. 6C, the sheet P1 is returned to the sheet sending path 12 and then sent to the conveying (sheet-discharging) rollers 6. Next, when the front end of the sheet P1 is released from the conveying rollers 7, as shown in FIG. 6D, the direction of rotation of the conveying rollers 7 is switched again, so that a sheet P2 conveyed next is pulled in the pull-in path 11. Further, the sheet P2 is handled in the same way as the sheet P1.
In the case where, in the both-side printing mode, first a toner image is recorded on the first surface (front surface) of the sheet P1, and then the sheet P1 is turned over and returned through the return path 14 to the image forming means to record a toner image on the second surface (rear surface), and thereafter the sheet P1 is turned over and discharged into the sheet discharge tray 9; as shown in FIG. 7A, the sheet P1 only on the first surface (front surface) of which an image has-been recorded is pulled in with the conveying rollers 7, and as shown in FIG. 7B, immediately after the rear end of the sheet P1 reaches the position B, the conveying rollers 7 are turned in the reverse direction, so that as shown in FIG. 7C, the sheet P1 is sent to the return path 14 until, as shown in FIG. 7D, it is released from the conveying rollers 7.
Accordingly, in the both-side print mode, when compared with the one-side print mode, the time required for turning over the sheet; that is, the sum of the time for the conveying rollers 7 to pull in the sheet to be turned over and the time required for turning the conveying rollers 7 in the reverse direction to release the sheet, additionally needs the time which corresponds to the conveyance of the sheet over the distance which is about twice the distance between the positions A and B.
As shown in FIGS. 7C to 7F, in order that the following sheet P2 (on both sides of which images have been recorded) is pulled in the conveying rollers 7, and turned over so as to be discharged into the sheet discharge tray 9, the distance between the two sheets P1 and P2 should be long enough to avoid the difficulty that the sheet P2 is lapped over the preceding sheet P1 at the conveying rollers 7. That is, in the case of the both-side print mode, it is necessary to add at least the distance which is about twice the distance between the positions A and B to the sheet distance in the one-side print mode.
Accordingly, in the both-side print mode, the printing speed must be decreased.
In order to overcome the above-described difficulty, the apparatus may be modified as shown in FIG. 5: That is, a sheet turn-over device 19 is provided between the image recording apparatus 1 and the sheet discharge tray 9, and a sheet turn-over operation to send a sheet to the return path 14, and a sheet turn-over operation prior to the sending of a sheet to the sheet discharge tray 9 are separately carried out. However, the modification results in an increase in manufacturing cost, thus being not always advantageous.